The molecular architecture of two enzymes involved in cyclic nucleotide metabolism purified from bovine brain cortex, adenylate cyclase and phosphodiesterase, will be investigated. The numbers of substrate, activator and antagonist ligand binding sites in these enzymes will be examined as well as their distribution among the polypeptide chains of the enzymes, and the interactions of these sites in modulating catalytic activity. In addition, the presence of the NAD-binding domain in the adenhypophyseal hormones will be investigated with respect to the nucleotide specificity of the domain, its distribution among the alpha and beta subunits and its physiological function. Finally the kinetics of the folding of an NAD-binding domain will be measured to determine whether its wide spread occurrence results from its comparative rapid folding into a stable structure.